(Anglais) Electroless nickel-boron coatings have several advantages over electroplated nickel and electroless nickel-phosphorous: they are harder than both other coatings and, as all electroless coatings, can be applied easily to complex shapes and all substrates, even non conducting ones, contrary to electroplated coatings. However, their high initial hardness, good adhesion and high wear resistance suggest that the ultrasound application would generate coatings with better properties than the existing ones.
Ultrasound assisted nickel electroplating has been extensively studied and some research was carried out about ultrasound assisted electroless nickel-phosphorous plating.
The addition of an acoustic field has been shown to promote beneficial effects on electrochemical processes in general and electroless deposition in particular. The deposition of Ni occurred in an aqueous bath containing a reducible metal salt (nickel chloride), reducing agent (sodium borohydride), complexing agent (ethylenediamine) and stabilizer (lead tungstate). Due to the instability of the borohydride in acidic, neutral and slightly alkaline media, pH was controlled at pH 12±1 in order to avoid destabilizing the bath. Deposition was performed in two different configurations: one with a classical mechanical agitation at 300 rpm and the other employing ultrasound at a frequency of 35 kHz. In addition, three different temperatures are tested; 95°C, 90°C and 85°C. The results showed that low frequency ultrasonic agitation could be used to produce coatings from an alkaline NiB bath and that the thickness of coatings obtained with ultrasound increased the when compared with baths produced in the same temperature without ultrasound. In the case of baths realized at 95°C the plating rate increases by over 50% compared to those produced using mechanical agitation. Corrosion characterization was performed by potentiodynamic polarisation curves in two different media 0.1M NaCl and 0.1M NaOH. Tribocorrosion properties were analysed in basic environment (0.1M NaOH) in passive condition.